Drug-induced hepatotoxicity is a major cause of new drug withdrawal from the market. It also limits further development of promising therapeutic agents even prior to clinical trials. Over-the-counter drugs are not exempt from hepatotoxic liability; for example, acetaminophen (ACP), a widely used (and misused) analgesic/antipyretic agent, when taken acutely in large doses, or chronically in greater than recommended dosages, can lead to liver and kidney damage. While individual pharmacogenetic profiles of hepatic cytochrome P-450 isozyme patterns, when correlated with chemical structures of the drugs and their possible metabolic activation pathways, hold promise as means to preclude susceptible subjects from drug exposure, the concept of therapeutic intervention or prevention methods have not yet attracted much attention, despite the fact that the standard clinical option for protecting the liver from ACP overdoses is to administer intravenous N-acetyl-L-cysteine (NAC) within 8 hours of the overdose (1). NAC, following deacetylation in the liver (2), provides L-cysteine, the sulfhydryl amino acid required for the rate-limiting first step in the biosynthesis of glutathione (GSH) (3). GSH is the body's natural defense against endogenously generated reactive oxidant species as well as reactive species such as NAPQI produced in the metabolism of ACP (4).
Experimentally, the administration of high doses of ACP to mice produces fulminant hepatic necrosis, manifested by quantum elevations in serum transaminase levels and histological evidence of centrilobular necrosis leading eventually to death. Post-administration of NAC, a prodrug of L-cysteine, or other cysteine prodrugs that have been sulfhydryl-modified, effectively protect mice against this ACP-induced hepatotoxicity (5, 6, 7).
Using a 14C-glycine/HPLC assay method to determine the extent of incorporation of the cysteinyl moiety of the cysteine prodrugs into GSH in rat lens (8), a radioactive peak near GSH was discovered which appeared to be produced metabolically. This substance was tentatively identified as the mixed disulfide of L-cysteine with GSH, viz., L-cysteine-GSH disulfide (CySSG). CySSG is produced endogenously via a thiol-disulfide exchange reaction between GSH and L-cystine (9), and possibly, the reaction of L-cysteine with GSSG (the oxidized form of GSH). CySSG, postulated to be a storage form of L-cysteine (10), has been detected in small quantities (relative to GSH) in liver and kidney samples from rats, but is present in comparable amounts as GSH, cysteine and cystine in rat and human plasma (11, 12).
Except for the monoesters (on the glycyl moiety) and the diethyl ester of GSH, prodrugs of GSH (13) have not been systematically investigated as protective agents against xenobiotic-induced hepatotoxicity.
Currently, there is a need for agents to treat cellular oxidative stress and to increase glutathione and L-cysteine levels in a cell. Such agents would also be useful to treat hepatotoxicity associated with the administration of other therapeutic agents.